1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) display device, and, more particularly, to a dual-type organic electroluminescent display device having two organic EL display panels.
2. Description of the Related Art
In general, electroluminescent display devices, which are spontaneous emission display devices for emitting light by exciting fluorescent organic compounds, have advantageous features such as a low driving voltage, and being thin and light. Also, much attention has been paid to electroluminescent display devices because they have advantageous features suitable for the next generation of display devices that can overcome several problems with LCDs, such as a wide viewing angle and a high response speed.
An organic electroluminescent display device (to be abbreviated as an organic EL device, hereinafter) basically operates as follows. When power is supplied, current flows while electrons migrate. At a negative electrode, the electrons move to an emitting layer through an electron transport layer. At a positive electrode, holes move to the emitting layer through a hole transport layer. The electrons and holes combine at the emitting layer, made of an organic material, to generate excitons having a high energy. The organic material constituting the emitting layer is capable of achieving a display of various colors, according to the organic electroluminescent material used.
In recent years, dual-type display devices have been increasingly demanded by users because they are capable of simultaneously displaying two picture screens in a folder-type electronic device such as a cellular phone.
For instance, referring to FIG. 1, a dual-type organic EL device includes a main organic EL display panel 10 and a sub organic EL display panel 20 coupled to each other so as to be viewed through the front and rear surfaces of the device.
In the dual-type organic EL device, the main organic EL display panel 10 includes a substrate 11, an organic EL portion 12 formed on the substrate 11, a cap 14 for protectively surrounding the organic EL portion 12, an absorbent 15 arranged in the cap 14, and a polarization plate 16 installed on one plane of the substrate 11. The sub organic EL display panel 20 coupled to the main organic EL display panel 10 has substantially the same configuration as that of the main organic EL display panel 10, and includes a substrate 21, an organic EL portion 22, a cap 23, an absorbent 24 and a polarization plate 25.
In the dual-type organic EL device, a user can selectively use information displayed on either the front panel or the rear panel, that is, either the main organic EL display panel 10 or the sub organic EL display panel 20. The dual-type organic EL device can be applied to, for example, a folder-type electronic device.
Another example of the dual-type organic EL device is shown in FIG. 2. Referring thereto, the dual-type organic EL device includes projections 32 and 42, of caps 31 and 41 of a main organic EL display panel 30 and a relatively smaller sub organic EL display panel 40, that are alternately disposed so that the overall thickness of the dual-type organic EL device is reduced.
In the dual-type organic EL device, when the cap 31 of the main organic EL display panel 30 contacts the cap 41 of the sub organic EL display panel 40, the organic EL device becomes bulky due to the heights of the cap 31 and 41 of the respective panels 30 and 40. Thus, there is still a limitation in thinning the dual-type organic EL device.
FIG. 3 shows still another example of a dual-type organic EL device, in which an organic EL display panel 50 and a liquid crystal display (LCD) panel 60 are coupled to each other.
Japanese Patent Publication No. 2000-58260 discloses a bilateral emission type EL device, which is configured such that a cathode electrode, made of a metal having a low work function, is interposed between two EL material layers, and respective transparent electrodes are installed on each of the upper and lower surfaces of the EL material layers. The disclosed device, however, has a problem in that arrangement of an absorbent for protecting the EL material layers against moisture is very difficult work.
Japanese Patent Publication No. Hei 10-255974 discloses a bi-directionally driven organic EL device. The disclosed organic EL device includes a pair of electrodes, at least one of which is transparent or semi-transparent, a host emission layer, which emits light by recombination of holes and electrons injected from both electrodes and made of an organic fluorescent material, and a pair of carrier transport layers disposed between the host emission layer and each of the electrodes, capable of transporting the holes and electrons injected from the respective electrodes and effectively confining the holes and electrons in the host emission layer, and which transmits light at the maximum area of the emitted light.
Also, Japanese Patent Publication No. 2001-332392 discloses a bilateral emission type EL device. The disclosed device includes organic layers having an electron transport layer, an organic emitting layer and a hole transport layer stacked between an opaque cathode and a transparent anode, and a transparent member provided at least at one portion of the opaque cathode as a light guide portion.
As described above, in the bilateral emission type organic EL device having the above-described configuration, an image is formed by a cathode and anode with organic layers interposed therebetween. Thus, a dual image display cannot be achieved.